Lasers operating with a wavelength near 2 microns are useful in many military and medical applications; they often utilize trivalent thulium ion (Tm3+) doped crystals as their optical gain medium. Traditionally, these are pumped by 0.7 to 0.8 micron light, exciting the Tm ions from the 3H6 ground state manifold to the 2nd excited state 3H4 manifold (FIG. 1, 105). Cross relaxation can result in a “two for one” excitation of the 3F4 upper lasing level, wherein one pump photon leads to two excitations, effectively doubling the otherwise 40% Stokes efficiency. This cross relaxation mechanism itself however manifests a <100% efficiency which is dependent upon the Tm3+ dopant concentration. Typical values for this cross relaxation mechanism are ˜70%, leading to an effective Stokes efficiency of ˜56% (=2×0.4×0.7). This leads to substantial waste heat, and thermal lensing, which is especially deleterious for high power applications.
What is needed, therefore, are techniques for more efficient generation of ˜2 micron light in Tm3+ lasers.